The physiological actions of prostaglandin (PG)E.sub.2 are mediated through interaction with the prostaglandin E receptor(s). There are three subtypes of the EP receptor, EP.sub.1, EP.sub.2 and EP.sub.3 (for review see Coleman et al., 1989). These three subtypes all show high affinity for PGE2 but show differences in their affinities for various agonists and antagonists and exert their actions through different secondary transduction mechanisms. Thus activation of the EP1 receptor is associated with a rise in IP3 and intracellular calcium, activation of the EP2 receptor results in a rise in intracellular cyclic AMP and activation of the EP3 receptor a fall in intracellular cyclic AMP followed by a rise in intracellular calcium. To date the only members of this family to be cloned are the mouse EP.sub.2 (Honda et al., 1993) and the mouse EP.sub.3.alpha. and EP.sub.3.beta. (Sugimoto et al., 1992; Sugimoto et al., 1993) subtypes. EP1 receptors are normally found on a wide variety of cells including the small intestine, kidney, stomach, muscle, eye, uterus and trachea, in humans and other animals. Binding of prostaglandin to the EP1 receptor protein elicits an increase in intracellular calcium levels. This signal causes the tissues to respond, for example, by muscle contraction.